How to deal with the climbing effect in viscosity measurement?

The climbing rod effect is one of the manifestations of high viscoelasticity of the polymer. For viscous fluids, the liquid level will be concave due to the centrifugal force

Shape (left); unlike viscous fluids, the polymer liquid (viscoelastic fluid) contained in the container, when placed in this sample

When the rotor is rotated, it does not smash toward the wall of the container due to inertia. Instead, it wraps around the rod and climbs up the rod.

The phenomenon of " crawling " (right), this phenomenon is called the Weissenberg effect, the normal stress effect, also known as the " envelope axis " effect.

The reason for this phenomenon is that the polymer material has elasticity, because the polymer fluid forms an anisotropic junction in the flow.

Constructed: a large molecular chain that has elasticity when rotated is oriented in the circumferential direction and undergoes tensile deformation, resulting in

The pressure toward the axis, the closer the off-axis is, the higher the shear rate, so the greater the normal stress, the corresponding, the polymer chain

The greater the elastic restoring force, the more the liquid is squeezed in the axial direction, and the climbing phenomenon occurs.

What is a viscoelastic fluid? It has the characteristics of viscous liquid and elastic solid.

Viscous liquid: stress, flow, and permanent deformation

Elastic solid: stress, deformation, removal of external force, deformation recovery

Viscoelastic fluid: deformation occurs when subjected to force, external force is removed, deformation is partially restored, and the longer the force is applied, the more the deformation recovery part

less.

The unfavorable consequences of the climbing rod effect and its solutions:

1 ,   The creeping effect is normal for polymer solutions, as long as the molecular weight reaches a certain number, it is necessary to climb, which is also high.

The embodiment of the flexibility of the molecular solution. When using a common viscometer to measure, due to the phenomenon of climbing poles, the data obtained is often larger than the real one.

Viscosity values, while such materials are actually pseudoplastic shear thinning, so the characteristics of such materials often give the tester a sense

To the confusion.

2 ,   To overcome the climbing effect:

For such materials, coaxial cylindrical rotors cannot be used. The simple method of zui is to adopt   RST-CPS   Cone and plate rheometer, according to

The viscosity range of the material is selected   CP25-1   or   CP50-1   Rotor, you can do a shear rate scan to get the basic rheology

Line, and then determine the measurement conditions after zui according to the situation , it is generally recommended to use low shear rate for measurement. As shown below, one

The rheological curves of the polymers under different polymerization conditions. It can be found from the curve that the material is a pseudoplastic fluid and has been experimentally tested.

Also found to have thixotropy, zui is determined at a lower shear rate (this sample is used   8   Measurement under the conditions of S-1 ),

Obtaining a stable and reliable viscosity value provides reliable experimental data for the determination of the production process.

For viscoelastic or gel-like samples (high polymerization and high concentration), the cone-plate rheometer is continuously linearly increased.

Adding and reducing the stress and shear rate of the sample can limit the elastic recovery of the internal structure to a certain extent.

Cause inconsistencies, so use   RST   The rheometer is more suitable for the test of the sample, the specific measurement conditions: can be taken

Use a shear rate scan to perform a rheological curve test, or use a lower shear rate ( 0-20)   S-1 ) for a while

The measurement is taken and averaged.

The results are very different (the user sample shows splashing, and the foreign sample can be produced normally).